Prior art office-based communications systems usually operate conventional fixed-line telephone units linked via an internal switchboard or PBX (private branch exchange.) Such fixed-line systems are able to provide relatively high voice quality. However, user mobility is severely impaired. It is also known to connect a base unit for a cordless system such as DECT to the internal PBX. This allows users to use cordless handsets in the office, but the server handsets (unless they are equipped with a dual-mode capability) can not be used outside the local cordless coverage area.
The improvement of digital cellular telephone technologies means that cellular telephone systems can now provide equivalent, if not higher, voice quality than fixed-line systems. Mobile systems also allow greater freedom of movement for the user within the office than do fixed-line systems. However, there can often be difficulties in receiving cellular telephone signals in an office.
RCP (Rich Call Platform) is a proprietary communications system developed by the applicants which introduces the concept of utilising mobile telephone units, such as conventional GSM mobile stations, in an office environment. The system preferably makes use of a known concept called Internet Telephony or Voice-over-IP (Internet Protocol).
Voice-over-IP is a technology which allows sound information to be transmitted over existing IP-based Local or Wide Area Networks or the Internet. In a similar way, data and video information can be encoded so as to be capable of transmission over the same networks. The technology thus provides for convergence and integration of three different media types over the same network.
Prior to the advent of Voice-over-IP, offices often operated three separate networks for the transmission of these media types. As indicated above, fixed-line telephone systems coupled to an in-house PBX provided for voice communication, an office-based LAN or Intranet (i.e. a packet-switched internal network), normally comprising computer terminals linked via network cards and under the control of a server station, provided for the transmission of “conventional” computer data, and video cameras linked to monitors via fixed line or remote transmission link provided for video communication.
Voice-over-IP effectively combines these three media types such that they can be transmitted simultaneously on the same packet-switched intranet network or IP-routed throughout the office environment and, using an external network such as the internet, beyond the confines of the office.
In order to provide for such media convergence, Voice-over-IP often uses a specific ITU (International Telecommunication Union) standard protocol to control the media flow over the Intranet. One common standard protocol used in Voice-over-IP systems, and the one used in the RCP system, is termed H.323.
H.323 is an ITU standard for multimedia communications (voice, video and data) and allows multimedia streaming over conventional packet-switched networks. The protocol provides for call control, multimedia management and bandwidth management for both point-to-point (2 end-users) and multipoint (3 or more end-users) conferences. H.323 also supports standard video and audio codecs (compression/decompression methods such as MPEG) and supports data sharing via the T.120 standard.
Furthermore, H.323 is network, platform and application independent allowing any H.323 compliant terminal to operate in conjunction with any other terminal.
The H.323 standard defines the use of three further command and control protocols:    a) H.245 for call control;    b) Q.931 for call signalling; and    c) The RAS (Registrations, Admissions and Status) signalling function.
The H.245 control channel is responsible for control messages governing the operation of the H.323 terminal including capability exchanges, commands and indications. Q.931 is used to set up a connection between two terminals. RAS governs registration, admission and bandwidth functions between endpoints and Mobile Telephone Servers (defined later).
For an H.323 based communication system, the standard defines four major components:    1. Terminal    2. Gateway    3. Mobile Telephone Server    4. Multipoint Control Unit (MCU)
Terminals are the user end-points on the network, e.g. a telephone or fax unit or a computer terminal. All H.323 compliant terminals must support voice communications, but video and data support is optional.
Gateways connect H.323 networks to other networks or protocols. For an entirely internal communications network i.e. with no external call facility, gateways are not required.
Mobile Telephone Servers are the control centre of the Voice-over-IP network. It is under the control of a Mobile Telephone Server that most transactions (communication between two terminals) are established. Primary functions of the Mobile Telephone Server are address translation, bandwidth management and call control to limit the number of simultaneous H.323 connections and the total bandwidth used by those connections. An H.323 “zone” is defined as the collection of all terminals, gateways and multipoint-control units (MCU-defined below) which are managed by a single Mobile Telephone Server.
Multipoint Control Units (MCU) support communications between three or more terminals. The MCU comprises a multipoint controller (MC) which performs H.245 negotiations between all terminals to determine common audio and video processing capabilities, and a multipoint processor (MP) which routes audio, video and data streams between terminals.
The conventional Voice-over-IP system described herein above normally utilises standard fixed-line telephone systems which are subject to the disadvantages outlined above, namely the lack of mobility and the lack of user commands.
The RCP concept takes Voice-over-IP further in that it provides for the use of conventional mobile telephone units, such as GSM mobile stations, within the Voice-over-IP system. To provide for such mobile communications within an intra-office communication network, RCP combines known Voice-over-IP, as described above, with conventional GSM-based mobile systems.
GSM base stations are provided to give coverage within the office, and are connected to the company's intranet. Intra-office calls to or from cellular telephones in the office are routed through the office intranet and extra-office calls are routed conventionally through the GSM network. Such a system provides most or all of the features supported by the mobile station and the network such as telephone directories, short messaging, multiparty services, data calls, call barring, call forwarding etc. RCP, therefore, provides for integrated voice, video and data communications by interfacing an H.323-based voice-over-IP network with a GSM mobile network.
The RCP system is a cellular network, similar to the conventional GSM network and is divided into H.323 Zones as described above. One H.323 Zone may comprise a number of cells. Two or more H.323 zones may be contained within an administrative domain. The allocation of H.323 zones to an administrative domain is an issue primarily concerning billing and is therefore not relevant to this invention.
A company RCP may be physically located in two or more separate office sites. These sites may reside in two different countries in areas managed by two or more different GSM operators. They may also reside in different regions of a country, in which two different GSM operators would be competing for customers.
It would be desirable to provide a method of allowing subscribers to use the intranet when visiting different sites belonging to the same company and to make calls from their own site to other company sites.